1. Field of the Invention
The present invention relates to the production of zinc pellets for use in a refuelable zinc-air battery.
2. Description of Related Art
Batteries having zinc electrodes may be constructed to consume particles of zinc metal. Zinc particle electrodes have the advantage of high active surface area per unit cross-section of the cell, which allows higher average current densities without excessive polarization. The particles may be in a stationary packed bed that is in continuous contact with a current collector, or may form a flowing slurry where the particles make intermittent contact with a current collector. In both cases, an electrolyte solution provides ionic conduction and carries away heat and reaction products. The electrolyte may also function as a reactant or to provide reactants. The electrolyte solution is typically an alkaline electrolyte such as potassium hydroxide, lithium hydroxide, sodium hydroxide, or a mixture thereof. (See Cooper, U.S. patent Ser. No. 08/152,964, now U.S. Pat. No. 5,434,020).
Electric vehicles making use of zinc-air batteries require (for economic reasons) a recycling of battery reaction products to form new zinc particles. On discharge, the zinc particles produce electricity and a reaction product consisting of zinc oxides, zincate ions, and zinc hydroxides. These reaction products can be converted back into zinc metal by cathodic reduction (electroplating or electrowinning). However, the zinc must be recovered in the form of particles of a size that can be used in the battery. If the particles are too large, pumping is difficult and the resultant electrode has a low active surface area. If the particles are too small, then the porosity of the resultant bed will be too low for sufficient electrolyte flow, and reaction products will clog the bed.
One type of zinc-air battery currently available cannot be recharged, but uses battery cassettes that are removed from the vehicles for charging. The zinc plates must be replaced and the spent, oxidized plates are reprocessed. Another conventional zinc-air battery is charged by connecting it to a recharger, which takes six hours for a full charge in the laboratory.
Thus, it is desirable to recover the zinc metal from spent electrolyte in the form of particles of substantially pure zinc having controlled and uniform size. The zinc particles may be transferred into a battery in the form of a pumped slurry--a technique called "hydraulic refueling." This invention provides a method for producing particulate zinc) and compressing the zinc into pellets having reproducible and uniform chemical, morphological, and physical properties.